The present invention provides a process for the production of boehmite crystals of a size and a shape suitable for use as a pigment/filler in paper, paint and ink applications.
It is known that boehmite is the thermodyamically stable phase of alumina monohydrate at temperatures greater than 100.degree. C. in liquors of the well-known Bayer process for the production of alumina from bauxite. Accordingly, boehmite can be precipitated at temperatures above 100.degree. C. from these caustic aluminate liquors. Although the initial rate of boehmite precipitation increases with increasing temperature, the yield may decrease since the degree of supersaturation with respect to alumina decreases.
The greatest degree of boehmite precipitation has been shown to occur in Bayer liquors under hydrothermal conditions (i.e., conditions of elevated temperature and pressure) of about 140.degree. C. when employing a caustic aluminate liquor having a molar ratio of 1.3, a retention time in the autoclave of 4-12 hours and a large seed charge, 260-340 g/l of boehmite crystals.
The term molar ratio can be defined as follows: EQU (Moles Caustic in Solution)/(Moles Alumina in Solution) = (1.645).times.[g/l Na.sub.2 O.sub.free ]/[g/l Al.sub.2 O.sub.3 ]
In the above equation, Na.sub.2 O.sub.free is defined as the amount of caustic present as NaOH and combined with alumina.
For example, a Bayer liquor which contains 140 g/l of free caustic (expressed as Na.sub.2 O.sub.free) and 177 g/l alumina has a molar ratio of 1.3 using the above equation.
Alternatively, it is known that, depending upon the liquor composition, under atmospheric pressure conditions, and at temperatures of up to about 140.degree. C., crystalline boehmite can be precipitated from Bayer liquors. This is accomplished using 100 g/l of a very fine gel-type boehmite seed (such as CATAPAL, made by CONOCO INC.), a 1.4 to 1.5 molar ratio and a retention time of 6 hours in the autoclave, as described for example in the West German Patent application of C. Miara et al., DE 3528534A1 (Aug. 8, 1985).
However, neither of the above-mentioned methods are suited to the production of fine boehmite crystals for use in pigment/filler applications because the products are too coarse. Moreover, despite the fact that both methods are seeded processes, they provide relatively low yields of boehmite in relation to the amount of seed used, and also in relation to the high alumina content of the starting liquor. Consequently, the liquors must be treated further, so that after separation of boehmite, the remaining alumina concentrations can be lowered to economical levels.
As described in copending U.S. Pat. Application Ser. No. 847,634,now U.S. Pat. No. 4,668.486, a Bayer spent i.e., a liquor having a high Na.sub.2 O.sub.free /Al.sub.2 O.sub.3 molar ratio and a low alumina content, when subjected to organics oxidation (i.e., a process for removing from the Bayer liquors organic contaminants present in the original bauxite used) in the presence of copper catalyst ions, can be induced to precipitate about 50% of its alumina content as copper-containing boehmite. This process is conducted under hydrothermal conditions (increased pressure and temperature) and the copper-containing boehmite is recycled as a catalyst/catalyst carrier system. In addition, no seeding of the liquor is necessary for boehmite crystallization. Moreover, the retention time requirement (the amount of time that the liquor to be oxidized must remain within the autoclave) for the combined organics oxidation - boehmite crystallization is of the order of one hour or less. However, the boehmite particles precipitated by this process are not suitable for paint, ink and filler applications because of their coarse size (5-15 microns).
As mentioned above, none of the prior art methods for precipitating alumina can be used to produce fine boehmite crystals for use in a pigment/filler application. Therefore, what is needed is a novel process for the production of such boehmite crystals. The present inventor has unexpectedly found that if the organics oxidation of a Bayer caustic aluminate liquor is carried out in the absence of copper ions and if the hydrothermal boehmite crystallization is carried out under conditions of vigorous agitation, then the resulting boehmite crystals have a particle size and form that renders them well- suited for use as pigments and/or fillers in paints, papers, inks and the like. Advantageously, using the method of the present invention, the boehmite is crystallized out from an organics-oxidized (i.e decontaminated by breakdown of organic carbon contaminants) Bayer liquor which is low in organic carbon content and hence, relatively colorless.
It is therefore an object of the present invention to provide a process for the production of a novel, fine, tabular, crystalline boehmite.
It is a further object of the present invention to provide novel boehmite crystals of a fine, tabular form.
A still further object of the present invention is to provide boehmite crystals having a particle size of between about 0.2 and 0.8 microns and essentially tabular (i.e., flat) shape.
As used herein, "tabular" is defined as flat crystals, i.e., crystals having one dimension substantially smaller than the other two dimensions. Substantially smaller is defined as having one of their average height, width and length dimensions to be less than about 50% of the other two dimensions. It should be noted that the tabular crystals can be square, diamond and hexagonal-shaped.
These and other objects of the present invention will be apparent to those of ordinary skill in the art in the light of the present description, accompanying claims and appended drawings.